This invention relates generally to methods for eliminating pick skew in a media handling subsystem, and more particularly, to a method for squaring a page at a drive roller using information sensed by a single emitter-detector pair.
A media handling subsystem transports a media sheet through a printing device, such as a computer printer, fax machine or copy machine. The media sheet is picked from a stack, then moved along a media path using one or more sets of rollers. Along the path the media sheet is positioned adjacent to a printhead which generates character or graphic markings on the media sheet. For proper placement of the markings, the position and alignment of the media sheet are known.
One source of misalignment occurs during a pick cycle. A pick cycle encompasses the steps of picking a single sheet from a stack of media sheets and moving the sheet away from the stack along a media path. For example, a pick roller often is used to drive a media sheet into one or more corner separators. Corner separators are flaps located on one or both leading corners of a media stack. The pick roller exerts a drive force causing a buckle in affected corners of the media sheet, allowing the sheet to pop over the corner separators and move forward. The drive force, however, is insufficient to create a buckle in underlying sheets, so that the top sheet is picked and moves past the underlying sheets. According to another example, a pick roller drives a media sheet into a separator pad. A separator pad is a friction pad into which a leading edge of the media sheet is driven. The pick roller exerts sufficient drive force for the top sheet to overcome the friction drag of the separator pad and move forward. The drive force on the underlying sheets, however, is insufficient to overcome the drag. Thus, the top sheet is picked and moves past the underlying sheets.
As the media sheet pops forward to separate from the stack, the media sheet may skew. This is referred to as pick skew. As the media sheet moves along the media transport path the rollers urging the sheet forward may cause additional skew. This additional skew is referred to as feed skew. The pick skew and feed skew, together with skew in the stack itself, are referred to as media skew.
If a media sheet is skewed, then the printout onto the media sheet will not be square to the page. The result is an aesthetically displeasing output alignment. One approach for addressing such problem is to detect media skew, then compensate for the skew when applying markings to the page. In effect the placement of markings is skewed an amount comparable to the media skew. As a result, the markings are placed square to the page--an aesthetically pleasing output alignment. A method for detecting such media skew is described in the above-referenced patent application, incorporated herein by reference. Compensating for media skew, however, places a burden on the print throughput. Markings from more than one line, for example, may have to be managed. As the page per minute print speed of a device increases such burden becomes significant. Accordingly, there is a need for another approach for handling skew. As pick skew and stack skew are substantial components of media skew, and because feed skew typically is insignificant, this invention addresses the problem of stack skew and pick skew.